In many mobile radio communications systems, transmit diversity is employed in order to create favourable conditions for the transmission of a radio signal from a radio base station to a user equipment. By transmitting a signal from two or more different antennas, more than one transmission path from the radio base station to the user equipment can be created, and the risk of loosing data due to fading etc. can hence be reduced. Furthermore, a desired signal beam shape can be achieved by the use of more than one antenna, if the antennas are placed at a sufficiently small distance from each other for the antennas to be correlated.
In a system employing transmit diversity, the radio base station generally transmits a pre-determined pilot signal from each of the transmitting antennas, which pilot signals are known by the user equipments. By measuring the received pilot signals, and comparing the measured pilot signals to the known transmitted pilot signals, a user equipment can determine a so called transmit weight for each of the transmitting antennas. A transmit weight is typically a complex number, indicating a phase and amplitude difference between the transmitted signal and the received signal, or between two different signals received by the user equipment. These transmit weights are then fed back to the radio base station, which can use the transmit weights in order to compensate for the differences in transmission path for the signal when travelling from the different transmitting antennas.
In EP1453223, a transmit diversity system comprising a plurality of antennas is disclosed. The plurality of antennas is divided into a plurality of antenna groups. A common pilot signal is transmitted via a reference antenna included in each group.